This invention relates generally to an implantable pacing lead and more specifically to a pacing lead having a high efficiency tissue stimulating and signal sensing porous electrode for use with a cardiac pacemaker and a method for making the porous electrode.
For a cardiac pacemaker, implant lifetime is determined by the energy delivered per pulse. The pacemaker will have a longer life if the energy delivered per pulse is maintained at a minimum. Alternatively, the energy can also be used to provide for more features in the pacemaker. The design of an implantable pacing lead which is used with the pacemaker is influenced by the optimum signal for pacing stimulation. Physiologically, a cardiac pacemaker must be capable of generating a signal with a sufficient magnitude to depolarize the excitable cells of the endocardium. The electrode size, material, surface nature, and shape; the body tissue or electrolyte conductivity; and the distance separating the electrode and the excitable tissue, combine to determine the energy required of the pacemaker. Accordingly, the main factors to be considered with regard to the design of implantable pacing lead's electrode are: the size, surface nature, material and shape; the fixation of the electrode to the tissue; and the endocardial tissue reaction.
In selecting a pacemaker, the current drain, and therefore the implant lifetime, is determined by the impedance to pacing pulses. The pacemaker lead's electrode must be capable of delivering a pacing pulse with a pulse width generally in the range of 0.01-2.0 milliseconds and 0.5 to 10.0 volts to the tissue, and to also sense and transmit a QRS signal arising in the atria and ventricles of the heart to the pacemaker circuitry. Generally, the electrode-electrolyte system impedance is higher for sensing than for pacing. Pacing leads for pacing and sensing in the atrium, which can exhibit different stimulation and depolarization parameters than the ventricle, are also required.
The electrode-endocardial tissue system impedance characteristics may be understood in terms of an interface component and a spreading resistance component. The interface component occurs within a few microns of the surface of the electrode. The spreading resistance component depends predominantly on the tissue resistivity. Generally, the former reflects the charge transfer characteristics of the electrode-tissue interface influenced mostly by the surface area and material of the electrode, and the latter reflects the overall size and shape of the electrode; the surface nature of electrode; and the resistivity of the tissue.
The current drain of a pacemaker is determined by the impedance of the pacemaker circuitry, the nature of the electrode lead resistance, and the characteristics of the electrode tip interface with the electrolyte system. For a given pacemaker circuit and electrode lead design, the current drain is well defined. Thus, the nature of the electrode-endocardial tissue interface determines the overall current requirements of the system.
As an additional design factor, the most significant frequency of the pacing pulse is on the order of 1 KHz. At this frequency, the interface impedance is small and most of the impedance to the pacing pulses is due to the bulk or spreading impedance. This is determined by the shape and size of the electrode tip and is generally inversely related to the radius of the electrode tip.
The most significant frequency components of a signal to be sensed, i.e., the ventricular QRS, are in the bandwidth of 20-100 Hz. In this region, the interface impedance of the sensed signal becomes the most significant. The interface impedance is determined in large part by the microsurface area of the electrode tip and develops within a few microns of the surface. As described herein, the microsurface area of a porous electrode tip is the wettable surface, area which includes all of the exposed and interstitial porosity surfaces of the electrode tip.
As a final design consideration, it has been determined that the pacing or stimulation threshold is a reflection of the electrical energy required for a pulse to initiate a cardiac depolarization. The stimulation threshold typically rises for a period of a few weeks after the implant of a cardiac pacemaker generally as a result of an increase in the spacing between the electrode and the excitable tissue. The increase occurs due to the development of a fibrous capsule around the electrode tip. The thickness of the fibrous capsule is generally dependent upon the mechanical characteristics of the distal end of the lead (i.e., stiff or flexible); the geometry of the electrode tip; and the microstructure of the electrode tip, such as a porous electrode surface and the electrode material itself. In this regard, the environment of the endocardium must be considered. Specifically, the constant beating of the heart can cause the electrode to pound and rub against the endocardium, causing irritation and a significant subsequent inflammatory response, which ultimately results in healing, and the development of a fibrotic tissue capsule about the electrode tip. Also, a rough surface microstructure or one with sharp protrusions for the electrode will tend to be abrasive or traumatic on the abutting heart cells, also causing irritation, which also tends to cause the development of a thicker fibrotic capsule.
In view of the above characteristics of an electrode and its implantology issues for a cardiac pacemaker, it is clear that an electrode tip with a small geometric surface area (resulting in higher pacing impedance) will have a low current drain. However, in order to enhance sensing, the same electrode tip should have a large microsurface area and be of such a material to result in a low polarization and high capacitance which, in turn, results in a low sensing impedance and improved sensing. A cardiac pacemaker electrode tip that is constructed to be porous is therefore preferred in order to best satisfy these requirements.
In a pacemaker electrode, minimal tissue reaction is desired around the tip, but firm intimate attachment of the electrode to the tissue is essential to minimize any electrode movement relative to the abutting tissue. A porous electrode tip with macro tissue entrapping structure allows rapid fibrous tissue growth into a hollow area or cavities in the electrode tip to facilitate and enhance attachment of the electrode to the heart. A reduced lead dislodgement rate is also expected as a result of such tissue ingrowth. A further aspect of importance is selection of porosity size, which must be such as to accommodate economical construction techniques, overall dimensional tolerances, and tissue response constraints.